The present invention relates to a slewing control device for a crane having a slewing body.
In a crane such as a hydraulic truck crane having a slewing body mounted on a traveling body and a multi-stage telescopic boom derrickably supported to the slewing body, a hoisting capacity of the crane varies with operational conditions such as a boom length, boom angle, outrigger expanded condition, and slewing angle. For example, when all of four outriggers of the truck crane are expanded at the maximum, the hoisting capacity can be desirably enhanced. However, when an expansion length of one or more of the outriggers is reduced according to a surrounding condition, the hoisting capacity in a slewing area corresponding to the reduced expansion length is reduced. Accordingly, it is necessary to limit a slewing range according to an expanded condition of each outrigger. Further, it is necessary to limit the slewing range so as to prevent a suspended load or the boom from contacting surrounding obstacles such as buildings. In this circumstance, it is demanded that slewing of the slewing body can be automatically stopped as required.
Conventionally, various devices for automatically stopping the slewing are known as follows:
(1) It is known from Japanese Patent Publication No. 60-20319, for example, that automatic stop of slewing is effected by setting a safe area and a dangerous area of slewing, outputting an automatic stop signal before the slewing reaches the dangerous area, and selecting an operational position of an electromagnetic closing valve by the signal to communicate a vent circuit of a main relief valve provided between a pump and a slewing direction selecting valve to a tank and thereby unload a discharge oil from the pump to the tank.
(2) It is known from Japanese Patent Laid-open Publication Nos. 62-31703 and 62-13619, for example, that a discharge pressure of a slewing motor is controlled by a variable relief valve or the like according to an inertia moment in braking and stopping the slewing, thereby controlling a braking torque.
(3) It is known from Japanese Utility Model Laid open Publication No. 2-18485, for example, that a discharge oil from the motor is unloaded in braking and stopping the slewing, and a discharge pressure of the motor is controlled by an electromagnetic proportional pressure control valve.
In the above prior art (1), when the automatic stop signal is input into the electromagnetic closing valve during the slewing, the discharge oil from the pump is unloaded to the tank. Accordingly, a pressure (accelerating pressure) on a suction side of a slewing motor can be made substantially zero. However, a pressure (brake pressure) on a discharge side of the slewing motor cannot be controlled. For this reason, if an operational position of the direction selecting valve is maintained at a slewing position, the slewing motor cannot be positively stopped but the slewing body continues to be rotated by inertia regardless of unloading the pump. Accordingly, it is necessary to mode the operational position of the direction selecting valve from the slewing position to a neutral block position, so as to positively stop the slewing body. If such a select operation is delayed, there is a danger that the slewing body will reach the dangerous area.
In the above prior art (2), the discharge pressure of the motor is controlled according to an inertia moment under the condition where a pressure oil to the motor is blocked upon braking of the slewing. However, although the discharge pressure of the motor can be controlled, the suction pressure of the motor cannot be controlled. Accordingly, a pressure differential between the discharge pressure and the suction pressure of the motor cannot be precisely controlled, with the result that it is difficult to stop the slewing body at a target position accurately.
Meanwhile, a slewing control system is classified into a neutral brake system wherein when the operational position of the direction selecting valve is returned to the neutral position, circuits on opposite sides of the slewing motor are blocked to stop the slewing and a neutral free system wherein when the operational position of the direction selecting valve is returned to the neutral position, the circuits on the opposite sides of the motor are communicated with each other to inertially rotate the motor (inertial slewing operation). In both the above prior arts (1) and (2), it is necessary to employ the slewing direction selecting valve, the device can be applied to the neutral brake system only.
In the above prior art (3), the discharge oil from the pump is unloaded upon braking of the slewing, and the discharge pressure of the slewing motor is variably controlled by the electromagnetic proportional pressure control valve. Accordingly, a pressure differential between the discharge pressure and the suction pressure of the slewing motor can be controlled more precisely than that in the prior arts (1) and (2), and the accuracy of braking and stopping can be made higher than that in the prior arts (1) and (2). Furthermore, the device in the prior art (3) can be applied to both the neutral brake system and the neutral free system. However, in the prior art (3), it has been found that when a total braking torque is small, there sometimes remains slight oscillation of a suspended load upon stoppage of the slewing body.
In braking the slewing, when the pressure differential between the discharge pressure and the suction pressure of the slewing motor becomes zero, the slewing (braking) torque becomes theoretically zero and the slewing motor is stopped to thereby stop the slewing body with no oscillation of the suspended load remaining. However, since there exists a peculiar braking torque due to an internal friction in the motor and an internal friction in a slewing speed reduction unit in a power transmitting system for slewing, a total braking torque (i.e., the sum of the peculiar braking torque and the hydraulic braking torque) cannot be completely zero in spite of the zero pressure differential. As a result, although the pressure differential is made zero, and the total braking torque is made apparently zero to stop the slewing, the oscillation of the suspended load is generated by the above mentioned remaining peculiar braking torque. Accordingly, in order to stop the slewing without leaving the oscillation of the suspended load, it is necessary to further reduce the total braking torque down to zero and make the pressure differential become smaller than zero.
None of the above prior art devices can control the brake operation so as to make the pressure differential become smaller than zero, that is, make the discharge pressure of the motor become lower than the suction pressure of the motor. In these circumstances, it is necessary to solve this problem.